(1) Field of the Invention
This invention relates to a method of making an expanded and crosslinked polyester resin composition, wherein a liquid unsaturated polyester composition catalyzed with a peroxide is mixed with a liquid borohydride blowing agent.
(2) Discussion of the Prior Art
The use of borohydrides, e.g. sodium borohydride (NaBH.sub.4) as blowing or foaming agents for plastics material is known. Such borohydrides are extremely active reducing agents and, as such, one molecule of borohydride will produce four molecules of hydrogen on reaction with hydroxyl compounds. EQU NaBH.sub.4 +2H.sub.2 O.fwdarw.NaBO.sub.2 +4H.sub.2
It can be seen that the volume of gas produced is very high in proportion to the amount of borohydride. Thus borohydrides are desirable as foaming or blowing agents.
However, in the past, the use of borohydrides for this purpose has been subject to some disadvantages.
Incoporation of a foaming agent into a resin mix must be of even distribution throughout the mix to achieve even foaming of regular cell size over the whole of the foam. For this reason, foaming agents are often incorporated in liquid medium into resin mixes using conventional processing equipment in which prechosen proportions of liquids are mixed and dispensed from the equipment as a uniform mixture. It has, however, not been possible to use this technique with borohydrides partly because there are few liquids into which they can be incorporated as dispersions or solutions without reaction. Even where solutions are possible the borohydrides are not very soluble and large quantities of solution would necessarily be used to incorporate sufficient borohydride into the resin mix. The large quantities of solvent involved would be detrimental to foam production and stability. While the above problems are general, they are of especial importance in the case of polyester resins which contain groups reactive with the borohydride. Thus, while the use of borohydrides with other resins has sometimes been found practicable, with polyesters it has been found that the fast speed of the reaction causes great problems. The use of aqueous solutions of borohydrides has been proposed but the presence of water causes degradation of the resin. Thus, while it can be seen that the use of borohydrides as foaming agents is desirable, especially as it also acts as a free radical supplier thus acting also to initiate gelling, many polyester resins are currently foamed without their use.
Many references to methods of preparing foamed polyester resin compositions using chemical blowing agents are available. Those of interest are: U.S. Pat. No. 2,461,761, U.S. Pat. No. 3,479,303, U.S. Pat. No. 4,028,289, U.S. Pat. No. 4,119,583, U.S. Pat. No. 4,122,047, U.S. Pat. No. 4,347,331. These disclose use of carbonates or bicarbonates which produce carbon dioxide when contacted with acids, to act as the blowing agent for foaming of the unsaturated polyester resin compositions.
U.S. Pat. No. 4,216,294, U.S. Pat. No. 4,358,548 discloses the use of isocyanates in conjunction with hydroperoxides to produce carbon dioxide and free radicals for expansion and crosslinking of the polyester resin compositions.
U.S. Pat. No. 3,920,590, U.S. Pat. No. 4,322,502 disclose a process of simultaneous generation of nitrogen for foaming and free radicals for crosslinking of the unsaturated polyester resin compositions by using sulphonyl hydrazides, hydrazines or hydrazones.
U.S. Pat. Nos. 4,028,344, 4,029,615, 4,101,464 disclose the use of azo compounds which produce nitrogen and free radicals on contact with acidic substances to simultaneously foam and crosslink the polyester resin composition.
U.S. Pat. No. 3,823,098, Jap. Pat. No. 75.06665 disclose the use of sodium borohydride aqueous solutions activated by the promoted and catalyzed unsaturated polyester resin compositions to produce hydrogen and free radicals for simultaneous foaming and crosslinking of the polyester resin composition.
At present, none of the chemical foaming agents meet all of the necessary criteria of a good commercial product such as good shelf life, good pot life, lack of toxicity, compatibility with standard processing equipment and ability to produce uniform, crack-free, fast curing foams of a wide range of densities from a wide range of unsaturated polyester resin compositions. Thus, in systems requiring additions of water or acid, the additions are deleterious to the curing and the strength of the cured product; the isocyanates require low hydroxyl content resins, i.e. special polyester resins for stability (shelf and pot life) of the resin component; azo compounds were found too toxic and the hydrazides, hydrazines and hydrazones are found to be inefficient foamers, i.e. a full range of densities in the foamed product is difficult to achieve at a reasonable cost.